Impedance unit



Feb. 14, 1939. J. c. BATCHELQR 2,147,138

IMPEDANCE UNIT Filed Jan. 17, 1955 INVENTOR.

Patented Feb. 14, 1939 UNITED STATES PATENT QFFICE 4 Claims.

My invention relates to an amplifier of the impedance-coupled type, and, more particularly, to such amplifiers as are designed to amplify a broad frequency spectrum such as is encountered 5 in the visual communication arts.

In the visual communication art, and more particularly in the television art, very broad bands of frequencies are encountered, and it is common that an amplifier for amplifying such sigl nals is called upon to amplify uniformly all frequencies from 20 cycles per second to one million cycles per second. It is known that ampliflers of the resistance-coupled type are adaptable to this service, but, were some special pre- 15 caution not taken, the amplification of the higher frequencies would be less than that of the lower frequencies because of the short-circuiting effect of the internal capacitances of the amplifier tubes used, and of the components of the ampli- 20 fier and the wiring between those components to the chassis or framework on which the amplifier is constructed. Inasmuch as the chassis is customarily at earth potential, I shall hereafter refer to these short-circuiting capacitances as 25 capacitances to ground.

In the design-of amplifiers for this unusually rigorous service, it is customary to consider each member of the circuit as a unit, and to attempt to minimize each capacitance to ground in order 30 to minimize the total leakage capacitance. For example, the coupling capacitor between the plate of one thermionic tube and the grid of the succeeding tube in a cascade amplifier is at a signal potential relatively high with respect to the chas- 35 sis or ground and an eifort is therefore made to maintain asmall physical size and a great physical isolation with respect to the chassis in order to maintain its leakage capacitance at a minimum value. Further, the plate and grid resistors are 40 also designed to be of small physical size and great physical isolation with respect to the chassis for the same reason.

When these precautions have been taken, and thermionic amplifier tubes have been chosen 45 which have very small internal capacitances,

present practice indicates that attempts to amplify frequencies over 250,000 cycles result in substantial decrease of amplification. In an effort to increase the value ofthe lowest frequency 50 at which amplification begins to decrease, there have in many such amplifiers been added inductive impedance units in series with the plate load resistance of the amplifier, and the increase of impedance of that inductance at the higher 55 frequencies presents to the plate of the amplifier tube a load which increases with frequency. Thus, in the absence of short-circuiting capacitances, the amplification of the amplifier would be increased; in the presence of such leakage capacitances, however, the amount of increase in the amplification per stage caused by the inductance may be offset by the leakage capacitances of the circuit components, and thus the overall amplification of the amplifier is maintained substantially constant to a higher frequency than would be possible in the absence of the impedance unit. In some case the amplification may increase with frequency up to a certain value, beyond which it decr ases.

The introduction such an inductance unit the piate circuit of resistance-coupled amplifiers therefore accompanied by some improvement in the range of frequencies which may be amplified, but an accompanying effect is present which reduces the total amount of improvement 20 which might be realized from the inductance. This effect is the additional capacitance to ground which the inductance unit itself introduces in the circuit, and this capacitance operates to reduce the im rovement which might otherwise be achieved.

With the foregoing in mind, it is an object of my invention to provide an amplifier of the resistance-coupled type having an added inductive impedance in the plate load of the tubes, in which the inductance and resistance are so combined that the leakage capacitance to ground of said combination is less than would be the capacitance to ground of those units in an uncombined form. This and other objects will be apparent from the following description of my in- Vention.

In accordance with my invention, I have provided a resistance-coupled amplifier of the conventional type in which is used a plate resistor of elongated cylindrical form, in series with which is connected an inductive impedance unit, and the inductance comprises a coil of wire wound upon the plate resistance unit as a core. With such a combination of inductance and resistance units, it is possible to realize a capacitance between the combined unit and the member upon which the unit is mounted of a value less than the sum of the capacitances of the units to the mounting were they not so combined.

In order to describe my invention more clearly, attention is directed to the accompanying drawing in which Figure l is a schematic circuit of an amplifier embodying my invention, and Figure 2 represents a plate impedance unit employed in my amplifier.

In Figure 1, input terminals l and 2 are provided whereby a signal to be amplified may be impressed through the input coupling capacitor E across the grid resistor it between the grid 25 and cathode 29 of the thermionic amplifier tube 8. The tube 3 is of the screen grid type, and suitable potential is impressed upon the screen grid 3i from the high potential battery 24 through the voltage dropping resistor 56 bypassed by the capacitor iii. The grid is suitably biased by the battery 22. The plate 2i of the tube 8 is connected ex ernally through the load impedance unit comprising an inductor i2 and resistor l3 to the battery which provides the operating potential for the plate El. The battery 26 is by-passed at the point where it connects with the resistor it by the by-pass condenser 2G.

The amplified signal which across the inductor i2 and resistor 5 is communicated through the coupling condenser t to the resistor It and is impressed between the grid 23 and the cathode 3&2 of the tube 3, which is also of the screen grid type. The screen grid 32 of the tube 9 is maintained at an appropriate potential by the battery 2% through the voltage dropping resistor ll which is lay-passed by the capacitor 8. The grid 26 of the tube 9 is properly biased by the battery 23. The plate 28 of the tube 9 is connected through the load inductor Hi and re sistor E5 to the battery 2 and the point of connection between the battery 2 and the resistance 55 is by-passed by the capacitor 25 The further amplified signal which appears across the load inductor it and resistor i5 is communicated through the coupling capacitor 7 to the output terminals 3 and d.

In Figure 2 the plate load impedance unit is shown in detail. A resistor 3 t comprising a cylindrical rod of carbon carried in any suitable binding material, is provided with the wire terminals 35 and 36, and may be painted with a waterproof insulating pai t Directly on the resistor 35 as a core are wound the coils 33 of a suitable size Wire such as number 23 B. 8i S. gauge silk covered copper wire so disposed and Wound as to produce a. coil having a minimum of distributed capacitance within the winding itself. The coils 33 are connected in series, inductively aiding, by the wires t5}, and one terminal 39 of the coil assembly is connected to one terminal 35 of the resistor 34. Thus a single unit is provided comprising a resistor and an inductor in series'having the terminals and 3?.

In certain embodiments of my invention I have found it suitable to provide a resistor 3% having a resistance of ten thousand ohms between the terminals 35 and and an inductance between the terminals 3: and 559 of 1.5 millihenrys. Such a coil, when wound directly on a resistor having a diameter of inch, may well have a distributed capacitance of less than one micromicrofarad and a direct current resistance of 35 ohms.

I have found that in amplifiers using a resistor and an inductor in the plate circuit, but these units not combined as in my invention, the capacitance to ground of those units respectively may be 10 and 8 micromicrofarads, but when the inductor is combined with the resistor in accordance with my invention and no other change is made in the amplifier, the effective capacitance to ground of the combined unit is appreciably less than 18 micromicrofarads.

It is apparent that various mechanical modifications are possible without departing from the spirit of my invention. For example, the coil may to advantage be wound as a single unit distributed along the surface of the resistor, and thus the capacitance to ground may sometimes be still further reduced without appreciably increasing the distributed capacitance within the coil. On the other hand, an inductor comprising a still larger number of sections than shown in Figure 2 may be used to advantage when the distributed capacitance consideration is predominant over the capacitance to ground efiect. Still further, the inductance unit may be wound on an insulating tube coaxial with and overlaying the resistor 3G for the purpose of insulating the coil from the resistor, physically isolating the coil from the resistor, for capacitance considerations or for the purpose of increasing the coil diameter for providing higher inductance.

Thus it is clear that many modifications of my invention are readily possible and such modifications should in no way be construed as limiting the scope of my invention. as set forth in the appended claims.

I claim:

l. An electrical impedance unit comprising an inductor wound upon a core having a magnetic permeability of the order of unity, said core being provided with a pair of electrical terminals and said core further having an electrical resistance between said terminals of more than 1000 ohms.

'2. An electrical impedance unit comprising an inductor wound upon a core having a magnetic permeability of the order of unity, said core being provided with a pair of electrical terminals and said core further having an electrical resistance between said terminals of more than 1000 ohms, one terminal of said inductor and one of said electrical terminals being electrically connected. I

3. An electrical impedance unit comprising an elongated, carbon-bearing, substantially cylindrical resistor and a plurality of inductively wound coils upon said resistor, said coils being connected in inductively aiding series relation, and said coils further connected in series relation with said resistor.

4. An electrical impedance unit comprising an elongated, substantially cylindrical resistor having a pair of electrical terminals, the resistance between said terminals being at least 10% ohms, a plurality of inductively wound coils upon said resistor, said coils being connected in inductively aiding series relation and having a total inductance of the order of 1.5 millihenrys.

JOHN C. BATCHELOR. 

